The present invention refers to injection molding machines for plastic materials, and in particular relates to a method and to a mold clamping press according to which use is made of a clamping device for clamping a mold, by which it is possible to operate with precision and at high speed.
In the plastic material injection molding, use is substantially made of two types of clamping presses for the closure of a mold, that is to say clamping presses having two platens, and presses having three platens provided with two or more peripheral tie-rods which extend longitudinally to the press.
In the case of clamping presses having three platens, these comprise a first stationary platen for supporting a half mold, a second movable platen for supporting the other one of the half molds, sliding along guide rods, and a third stationary platen forming part of a reaction structure, to which a toggle system is connected to control the sliding of the movable platen between an open condition and a closed condition of the mold, and the clamping of the same mold.
Presses of this kind require an expensive and bulky structure, and highly complex control systems necessary to generate the high clamping forces for tightly closing the mold, with mold closing and opening times compatible with the length of the entire operative cycle of the machine.
An alternative configuration to reduce costs and overall dimensions, contemplates the use of only two platens, including a stationary platen to support a first half mold, and a movable platen to support a second half mold, which is made to slide along horizontal guides by means of hydraulical actuators or other types of control systems.
A two platen press also comprises tie-rods which extend from one platen to the other one, and engage with mechanical clamping devices operated at the end of the working stroke during which the movable platen approaches the stationary platen, before exerting the high clamping force for closing the mold, or at the time of reopening of the mold at the end of each injection cycle.
Examples of injection molding machines with two platen clamping presses, and mechanical closing and clamping systems, are found in DE 1.159.137, U.S. Pat. No. 2,916,768, U.S. Pat. No. 5,188,850; U.S. Pat. No. 5,192,557 and in WO 98/51468.
Usually, in clamping presses of this type the tie-rods are rigidly fastened and protrude jutting out from one of the platens, in such a way that their extremities engage with clamping jaws or other clamping devices disposed on the other platen of the press; the clamping jaws must be operated beforehand to engage the tie-rods which subsequently are drawn by means of hydraulic actuators capable of exerting the necessary clamping force or tonnage for closing the mold.
The injection molding presses of the aforementioned type, present the drawback of requiring long rest times between different mold opening and closing operations, which increases the working cycle of the press.
In particular, pauses are required for operating the clamping devices; such pauses have significant repercussions on the output of this type of machine.
In fact, considering that in presses of this kind it is possible to operate with molding cycles in the range of 10-15 seconds, or times even less, it is clear that resting times in the range of 0.5÷3 seconds can prove to be unacceptable or of such length as to greatly conditioning the output of the machine. This depends mainly upon the mechanical and structural characteristics of the clamping devices, as well as upon the use of complex safety systems which must come into operation each time the press is to be open and closed.
U.S. Pat. No. 5,066,217, U.S. Pat. No. 5,417,913 and U.S. Pat. No. 6,186,770 show other types of two platen clamping presses.
In particular, U.S. Pat. No. 5,066,217 and U.S. Pat. No. 5,417,913 show a two platen presses whose tie-rods are axially movable with respect to the platens supporting the half molds, and in which use is also made of mechanical systems for locking the platens to the tie-rods, which substantially comprise radially movable half nuts, or angularly rotatable coupling members which engage with corresponding screw threads or annular ribs on the tie-rods of the press. Presses of this kind again present the same drawbacks inherent in the clamping and closing devices of the patents previously referred to; in particular, they present a greater complexity of the system for locking the platens to the tie-rods and intercycle times which are still considered high for this kind of press.
U.S. Pat. No. 6,186,770, which represents the closest prior art in respect to the present invention, also shows a two platen press in which the tie-rods are axially secured to one of the platens, and in which the other platen of the press is provided with a mechanical connection with each tie-rod, substantially comprising an internally threaded bush member which engages by screwing onto a screw threaded portion of the corresponding tie-rod, both in order to cause the sliding of the movable platen, and in order to generate the tonnage necessary for the tight clamping of the mold. A belt system is provided to synchronize the rotation of all the bush members.
According to a possible embodiment, the press comprises a first control means to cause the rotation of the bushes whenever the movable platen must be moved forward or backward, and second independent control means to rotate the bushes themselves, or alternatively the same tie-rods, whenever the tonnage for clamping the mold must be exerted. In both the solutions, in addition to generation of the tonnage for clamping the mold, the threaded bushes also serve to cause the sliding displacement of the movable platen, thereby resulting in frictional forces and stress between the screw threads.
It is therefore necessary to seek new and alternative solutions whereby it is possible to obviate the aforementioned drawbacks, for closing and securing the tie-rods to the platens supporting the molds, in two platen presses for injection molding machines.
Consequently, the main object of the present invention is to provide a method and a two platen press for injection molding, which make use of a locking device for the closure and locking of the platens to the tie-bars of a press, which is capable of obviating the aforementioned drawbacks.
In particular, one object of the present invention is to provide a method for clamping a two platen press, as referred to above, allowing extremely short clamping times, suitable for fast cycle machines, and which does not require any particular or complex safety system, thereby ensuring the utmost speed and precision during the closing and tight clamping steps of a mold.
A further object of the present invention is to provide an injection molding press of two platens type, provided with a clamping system for clamping the mold and securing the platens to the tie-rods which, in addition to possessing extremely short resting times, is also extremely functionally simplified in that the locking of the platens to the tie-rods and the tight clamping of the mold, take place simultaneously and in synchronism on all the rods of the press, without causing frictional forces.
A still further object of the present invention is to provide a clamping device of the type referred to above, by which it is possible to use separate control systems for the displacement of the movable platen and for locking at the same time the platens to the tie-rods, ensuring in all cases a fast and immediate clamping action whatever the final position of the movable platen with respect to the stationary one.
According to a general feature of the present invention, a method is thus provided for clamping a mold of an injection molding press of the type comprising:
a first platen for supporting a first half mold;
a second platen for supporting a second half mold, at least one of said platens being slidably movable with respect to the other one, parallel to a longitudinal axis of the press;
a plurality of peripheral tie-rods parallely extending to said longitudinal axis between the two mold supporting platens of the press;
first control means being provided for moving one platen towards and away from the other one, upon the closure and opening of the mold; and
second control means for the tie-rods, and threaded bush members parallely arranged to the longitudinal axis of the press, said bush members having a threaded portion operatively connectable to a threaded portion of said tie-rods, to provide a high clamping force to tightly close the mold, the method comprising the steps of:
supporting the tie-rods axially sliding with respect to both the platens of the press;
providing a locking bush, for each tie-rod, rotatably and movably supported in the axial direction, by one of the platens of the press;
each locking bush and the corresponding tie-rod including opposite interengaging screw threaded portions having an axial clearance between them;
moving one of the platens in respect to the other one for closing and respectively for opening the mold by actuation of said first control means maintaining the screw threaded portions of the locking bushes and the tie-rods frictionless disengaged by synchronously rotating the locking bushes;
interengaging the screw threaded portion of the tie-rods with the screw threaded portion of the locking bushes by causing an axial sliding movement of the bushes by said second control means, to urge a shoulder member of the bushes against a corresponding shoulder surface of the platen of the press; and
subsequently providing a clamping force to tightly close the mold, by generating a high axial force on the tie-rods and the locking bushes.
According to a second aspect of the invention, a locking device has been provided for locking a tie-rod to a mold supporting platen of a molding press said locking device comprising:
a displaceable tie-rod extending through the platen of the press, said tie-rod having an outer threaded portion thereon;
a locking bush rotatably supported by the platen, said locking bush having an inner threaded portion engageable and disengageable with the outer threaded portion of the tie-rod;
the threaded portions of the tie-rod and the locking bush being constructed and arranged to provide an axial clearance allowing a relative axial displacement between them;
an annular shoulder on the locking bush to be urged against a shoulder surface of the platen;
guide means rotatably engaging the locking bush to maintain the threaded portion of the same bush, disengaged from the threaded portion of the tie-rod, upon rotation of the bush and during displacement of the platen; and
control means to provide an axial displacement between the threaded portions of tie-rod and the bush, and to urge said annular shoulder against said shoulder surface at the clamping of the mold.
According to a further aspect of the invention, a press is provided for injection molding of plastic materials, comprising:
a stationary platen for supporting a first half mold;
a movable platen for supporting a second half mold, said platen being slidingly movable along a longitudinal axis of the press;
a plurality of peripheral tie-rods parallelly extending between the platens of the press, each of said tie-rods having a threaded portion at one end;
first control means being provided to displace the movable platen towards and away from the stationary one; and
second control means for the tie-rods and threaded bushes having threaded portions operatively engageable with threaded portions of the tie-rods to generate a clamping the force for tightly close the mold;
the tie-rods being movably supported in the axial direction by the platens of the press, through locking bushes rotatively supported by one of the platens of the press;
the threaded portions of each locking bush and the respective tie-rod having an axial clearance between them, to allow a relative axial movement of the tie-rod in respect to the locking bush;
each locking bush comprising an annular shoulder to engage a shoulder surface of a platen, by a short axial movement of the same bush; and
means for providing a synchronized rotation of the locking bushes correlated to the sliding displacement of the movable platen of the press, and to maintain the threaded portion of the bush disengaged from the threaded portion of the corresponding tie-rod;
said second control means provided for selectively causing a first sliding movement of the tie-rods with respect to the locking bushes, along said axial clearance to bring the screw threaded portions into contact with each other, respectively a second sliding movement of the tie-rods and the locking bushes to bring the annular shoulders of the bushes into engagement with the shoulder surfaces of a platen, and to tightly clamp the two half molds.